﻿// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "h264_bit_reader.h"
#include "macros.h"
namespace media {
    
    H264BitReader::H264BitReader()
    : data_(NULL),
    bytes_left_(0),
    curr_byte_(0),
    num_remaining_bits_in_curr_byte_(0),
    prev_two_bytes_(0),
    emulation_prevention_bytes_(0) {}
    
    H264BitReader::~H264BitReader() {}
    
    bool H264BitReader::Initialize(const uint8_t* data, off_t size) {
        DCHECK(data);
        
        if (size < 1)
            return false;
        
        data_ = data;
        bytes_left_ = size;
        num_remaining_bits_in_curr_byte_ = 0;
        // Initially set to 0xffff to accept all initial two-byte sequences.
        prev_two_bytes_ = 0xffff;
        emulation_prevention_bytes_ = 0;
        
        return true;
    }
    
    bool H264BitReader::UpdateCurrByte() {
        if (bytes_left_ < 1)
            return false;
        
        // Emulation prevention three-byte detection.
        // If a sequence of 0x000003 is found, skip (ignore) the last byte (0x03).
        if (*data_ == 0x03 && (prev_two_bytes_ & 0xffff) == 0) {
            // Detected 0x000003, skip last byte.
            ++data_;
            --bytes_left_;
            ++emulation_prevention_bytes_;
            // Need another full three bytes before we can detect the sequence again.
            prev_two_bytes_ = 0xffff;
            
            if (bytes_left_ < 1)
                return false;
        }
        
        // Load a new byte and advance pointers.
        curr_byte_ = *data_++ & 0xff;
        --bytes_left_;
        num_remaining_bits_in_curr_byte_ = 8;
        
        prev_two_bytes_ = ((prev_two_bytes_ & 0xff) << 8) | curr_byte_;
        
        return true;
    }
    
    // Read |num_bits| (1 to 31 inclusive) from the stream and return them
    // in |out|, with first bit in the stream as MSB in |out| at position
    // (|num_bits| - 1).
    bool H264BitReader::ReadBits(int num_bits, int* out) {
        int bits_left = num_bits;
        *out = 0;
        DCHECK(num_bits <= 31);
        
        while (num_remaining_bits_in_curr_byte_ < bits_left) {
            // Take all that's left in current byte, shift to make space for the rest.
            *out |= (curr_byte_ << (bits_left - num_remaining_bits_in_curr_byte_));
            bits_left -= num_remaining_bits_in_curr_byte_;
            
            if (!UpdateCurrByte())
                return false;
        }
        
        *out |= (curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_left));
        *out &= ((1u << num_bits) - 1u);
        num_remaining_bits_in_curr_byte_ -= bits_left;
        
        return true;
    }
    
    off_t H264BitReader::NumBitsLeft() {
        return (num_remaining_bits_in_curr_byte_ + bytes_left_ * 8);
    }
    
    bool H264BitReader::HasMoreRBSPData() {
        // Make sure we have more bits, if we are at 0 bits in current byte and
        // updating current byte fails, we don't have more data anyway.
        if (num_remaining_bits_in_curr_byte_ == 0 && !UpdateCurrByte())
            return false;
        
        // If there is no more RBSP data, then |curr_byte_| contains the stop bit and
        // zero padding. Check to see if there is other data instead.
        // (We don't actually check for the stop bit itself, instead treating the
        // invalid case of all trailing zeros identically).
        if ((curr_byte_ & ((1 << (num_remaining_bits_in_curr_byte_ - 1)) - 1)) != 0)
            return true;
        
        // While the spec disallows it (7.4.1: "The last byte of the NAL unit shall
        // not be equal to 0x00"), some streams have trailing null bytes anyway. We
        // don't handle emulation prevention sequences because HasMoreRBSPData() is
        // not used when parsing slices (where cabac_zero_word elements are legal).
        for (off_t i = 0; i < bytes_left_; i++) {
            if (data_[i] != 0)
                return true;
        }
        
        bytes_left_ = 0;
        return false;
    }
    
    size_t H264BitReader::NumEmulationPreventionBytesRead() {
        return emulation_prevention_bytes_;
    }
    
}  // namespace media
